feat(processing): add canonical_id.py -- SHA-256 ID generation

Single source of truth for stable entity IDs, matching
make_id() in graph/loader.py for full compatibility.
Helper functions for politician, company, contract, NGO ID generation.
Pure ASCII.

processing/canonical_id.py

@@ -0,0 +1,46 @@
+"""
+BharatGraph - Canonical ID Generation
+Single source of truth for deterministic entity IDs.
+Every node in the graph derives its ID from its most stable
+real-world identifier (CIN, constituency+name, etc.).
+Identical to make_id() in graph/loader.py for full compatibility.
+Pure ASCII.
+"""
+import hashlib
+import re
+
+
+def canonical_id(*parts) -> str:
+    """
+    20-char SHA-256-derived ID from canonical entity properties.
+    Same algorithm as make_id() in loader.py for full compatibility.
+
+    Examples:
+        canonical_id("narendra modi", "gujarat")        -> stable ID
+        canonical_id("L51100GJ1993PLC019067")            -> company by CIN
+        canonical_id("adani enterprises", "gujarat")    -> same every run
+    """
+    combined = "|".join(str(p).lower().strip() for p in parts)
+    return hashlib.sha256(combined.encode()).hexdigest()[:20]
+
+
+def canonical_id_for_politician(name: str, state: str,
+                                  election: str = "") -> str:
+    return canonical_id(name, state, election)
+
+
+def canonical_id_for_company(cin: str = "", name: str = "",
+                               state: str = "") -> str:
+    if cin:
+        return canonical_id(cin)
+    return canonical_id(name, state)
+
+
+def canonical_id_for_contract(order_id: str) -> str:
+    return canonical_id(order_id)
+
+
+def canonical_id_for_ngo(darpan_id: str = "", name: str = "") -> str:
+    if darpan_id:
+        return canonical_id("ngo", darpan_id)
+    return canonical_id("ngo", name)